Gosh, that's dumb. One would have had to keep Joseph Ratzinger/Benedict under strict surveillance from puberty to the present moment; or at least from his vow of celibacy to the present moment.
It's like trying to prove "'Shots' has never voted for a Socialist Workers' Party candidate." If you assert this, there's no way to disprove it. And hey, I'm willing to believe it. Stands to reason.
Not as dumb as your stating he was celibate in the first place!